Abstract
Most passive heat exchanger tube inserts only raise the Nusselt number by dramatically increasing friction, and studies on fin inserts have generally focused on low turbulent Reynolds numbers used in plant equipment. In this study, we numerically investigate a ram-horn internal fin whose branch radii follow a Fibonacci spiral, implemented in a 25 mm inner-diameter copper tube and solved using computational fluid dynamics software (ANSYS FLUENT 2026) with conjugate heat transfer for Re = 10,000–100,000. The fin number (N = 4, 6, 8) and stem height–diameter ratio (s/D = 0.06, 0.08, 0.1) were individually varied to examine their effects on the Nu, f, and heat retention values. Water was used as the working fluid, entering the tube at 350 K and 4 bar, and a 50,000 W/m2 outer-wall flux was imposed. Mesh independence and turbulent viscosity ratio sensitivity analyses were performed, with the results favoring the SST k–ω turbulence model. The plain tube sat close to the Petukhov friction factor correlation and agreed well with the Dittus–Boelter correlation, validating the plain pipe case. The results indicate that the proposed horn-like fins pin coherent vortices to the wall, increasing the near-fin turbulent kinetic energy. The thermal performance factor peaked at 1.46 (N = 6, s/D = 0.06, Re = 80,000), where the Nusselt number was 110% above that of the plain tube; heat retained by the fluid also rose by 14%. Constant pumping power analysis showed a 28% maximum heat transfer improvement when all finned pipe cases were examined. A multiple regression analysis yielded a correlation equation between Nu, Re, Pr, s/D, and N, which achieved R2 = 0.992.